The present invention relates to a method and apparatus for the production of polypropylene yarn.
1. Field of the Invention
2. Description of the Related Art
Fibers or filaments made from polypropylene have been available from the early 1960's when the resin first became commercially available. The first polypropylene filament yarn was produced on conventional melt spinning equipment used for the production of nylon and polyester yarns. Initially a two-stage process was employed where the polypropylene was melt spun through a spinnerette, cooled in a stream of air, and then wound onto bobbins in the undrawn state. This undrawn yarn was then processed on a draw twister machine where it was stretched between heated godet rollers and then immediately twisted up on a ring twisting spindle. On such a machine the final yarn speed would be in the order of 50-100 meters per minute.
Polypropylene yarn produced by this method displayed medium to good strength with low elongation which made it suitable for some industrial applications. The main problem with the commercial development of this method of manufacture was due to the extremely high cost of draw twisting machines, plus a labor intensive operation with very low output.
A modified version of the Draw-Twister, known as a Draw-Winder, where the ring twisting spindle is replaced by a high speed winding unit, enables a considerable increase in operating speed. Although the increase in speed offsets the capital cost and labor intensity, the yarn produced at higher speed showed a considerable reduction in strength, with corresponding higher elongation. Polypropylene yarn with these characteristics is perfectly acceptable for domestic textile applications but it is unsatisfactory for industrial use where strength is a prime requirement.
During the 1960's, a new development was taking place in the synthetic fiber industry and a new process evolved known as the "Spin-Draw-Wind" process. In this process the yarn is melt spun through a spinnerette, cooled in a stream of air, and then immediately stretched between heated godet rolls and finally wound up on on a high speed winding machine. In this process yarn speeds of 3000 meters per minute or more can be achieved. However, polypropylene yarn produced on this system has a relatively low strength and high elongation. These properties are again acceptable for many domestic textile applications but are of no use for industrial applications where strength is a prime requirement.
During the 1970's a new process was specially developed for processing polypropylene. This is known as the "Short Spin" process. In this process a large volume of cooling air is blown at relatively high velocity, typically 10 meters per second, across the face of the spinnerette onto the molten filaments as they emerge. The filaments are rapidly cooled and solidify within a few centimeters of the face of the spinnerette. These filaments are then stretched in hot air or in a steam chest between a series of godet rollers.
The Short Spin method was primarily developed for the production of staple fiber for use in carpets and floor coverings, and as such, requires very low strength and high elongation.
A development of the Short Spin process was to produce continuous filament yarn by replacing the staple cutting equipment at the end of the production line, with suitable winders. By carefully adjusting the processing conditions it is possible by this method to produce yarns with medium to good strength, but the adjustment is extremely critical and a considerable variation in physical properties can be experienced even between yarns produced under identical conditions on the same machine. Variation of strength also occurs along the length of a single yarn. Such uncontrollable variation makes yarn produced by the Short Spin process unacceptable for critical industrial applications.
For the Short Spin process to work, the polymer used must have high melt strength to withstand the force of the cooling air blowing on the molten filaments at the face of the spinnerette. Polymers which display high melt strength are those which have a wide molecular weight distribution. It is because of the variation in molecular weight distribution that the resultant fibers display an unacceptable variation of physical properties.
To produce a polypropylene yarn with consistent physical properties, it is necessary to use a polymer with a narrow molecular weight distribution. These polymers are produced by controlling the rheology during the polymerization process. However the effect of this is that such polymer has a relatively low melt strength making it at best difficult, and generally impossible to use on the Short Spin process.
In view of the foregoing, it is obvious that there was no satisfactory viable process for the production of consistently high quality, high strength polypropylene yarns for use in industrial applications. It was for this reason that some 30 years after polypropylene had become commercially available, only a minute quantity had found acceptance in industrial applications, in spite of polypropylene's other properties such as immunity to attack by most solvents, and chemicals in general.
It is well-known that the tensile strength of a filament yarn is directly related to the degree of molecular orientation in the fiber. It is further known that the total amount of molecular orientation is the sum of the orientation that is imparted during the melt spinning phase, i.e. as the polymer flows through the holes in the spinnerette, and as it is drawn away from the face of the spinnerette under tension prior to the polymer changing from the molten state to the solid state, plus the orientation imparted during the final drawing stage.
It is also well known that by utilizing known processes very slowly, high tenacity yarns can be produced but at such a rate as to be totally unviable in a commercial sense.